Titin is a 3000-4000 kD protein found in heart and skeletal muscle, and it has been proposed to play a major role in controlling the resting or passive tension in these tissues. Major developmental changes have been found to occur in cardiac titin as a result of alternative splicing pathways. A unique rat strain has been discovered with an autosomal dominant mutation that leads to a delayed fetal-to-adult titin transition pattern. These animals will be used to test the role of titin in cardiac hemodynamics in the intact animal using echocardiography and pressure-volume relationship measurements. Mechanical experiments on single cardiomyocytes will test hypotheses regarding titin's role in rest tension and the Frank-Starling relationship. Interactions betweer_ positively charged PPAK peptides from titin's extensible PEVK region with negatively charged polyE peptides will be tested using chromatographic, circular dichroism, electrophoretic, and surface plasmon resonance techniques. Studies will be designed to determine if binding of the PPAK and polyE peptides is specific to adjacent regions in the sequence or if multiple types of interactions can occur. Nontitin peptides rich in glutamic acid will also be examined for binding. Previously proposed phosphorylation sites will be identified in expressed titin polypeptides, and these sites will also be assayed for )hosphorylation state in intact fetal and adult titins. These experiments will test the hypothesis that certain _ites must be phosphorylated for proper assembly and/or function. Gene mapping studies will be conducted to localize the mutation site leading to the delayed developmental titin isoform program. The proposed studies will provide new information regarding the structure and function of this giant protein and its relation to human health and cardiovascular disease. The work may also provide novel insights on mechanisms of alternative splicing, an area of increasing importance in understanding the proteome. [unreadable] [unreadable]